Is it true that guard cells control the opening and closing of the stomates?
Guard cells, shaped like little kidney beans, are arranged in pairs at a stomate, with their concave sides facing each other. These concave sides have a thicker cell wall than the remainder of each cell’s wall, so that when water is driven in, the guard cells swell, causing the concavities to spread apart in the middle, opening an eye-shaped slit. This is the open stomate, which lets out gasses from respiration (namely O2, and often a characteristic fragrance, like Sweet Gum trees make the woods smell sweet) and water vapor. When the turgor pressure (or internal water pressure in each cell) subsides, the cells reduce in size, and the erstwhile separated concavities of the guard cells now fully touch, closing the stomate.
